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Post subject: What's the actual function of the 12ax7 tube in the SCX2?
Posted: Sat Dec 15, 2012 4:08 pm
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Is it actually for the preamp, a phase inverter, or part of the power section? I've read various things but I don't know enough about tubes (or the technology used in the amp) to know for sure. The only reason I asked is that I ordered some new JJ's, 12ax7's for both my SCX2's and didn't know if they needed to be matched, balanced, or anything else. I decided to just go for matched triodes and current balanced ( a $4.50 upgrade) but it seems like a small price to keep my amps happy.

I've never been to fussy about tubes but after I replaced the ones in my TRRI (also Groove Tubes) I was amazed... I'm hoping the new tubes benefit the tone of the SCX2's as well. Now granted, the tubes in my Twin were pushing eight years old but I'm also considering an Eminence Little Buddy for my combo... we'll see if it's in the holiday budget! 8)


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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Sat Dec 15, 2012 6:10 pm
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Let me try what I believe is new found understanding. There are only 3 tubes in the SCX2. Two power 6v6's and one preamp tube, a 12ax7. The 6v6's need biasing if replaced, but the 12ax7 can just be swapped out. I am going with a tung-sol for mine. I like the the musicality of it and it is loud with a very well pronounced reproduction of the frequencies. It a cleaner than many. For my ear the electro-harmonix were not good. Tone is personal, so it like picking a color to wear. JJ's have a bluesy/rock quality to them. I can see why they are popular. My preference is for something a little more hi-fi while still having some "attitude". I am thinking of getting 12ax7 and a 12at7. The latter is not as potent so early. It is also not as loud overall. It stays cleaner a little longer on the way up in volume and thereby lends a different tonal coloration that compliments what I like about the tung-sol to begin with. To my ears there is a hint of Twin to it.

I hope that answers your question. I think the info is correct. Hopefully there will be watchful eyes from certain members here.

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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Mon Dec 17, 2012 6:59 am
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Thanks Modwiz, so the 12ax7 is a tube for the preamp section of the amp? I'd read somewhere where the preamp was all DSP and that the 12ax7 functioned in some other capacity. Anyone else have any thoughts or knowledge in this department?


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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Mon Dec 17, 2012 9:32 am
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JamGuy wrote:
Thanks Modwiz, so the 12ax7 is a tube for the preamp section of the amp? I'd read somewhere where the preamp was all DSP and that the 12ax7 functioned in some other capacity. Anyone else have any thoughts or knowledge in this department?


My focus and discussion mainly pertained to channel one. This channel is my favorite and well worth the price of the amp to me. There is a real magic for me with this amp. The digital effects seem to blend nicely for me. Basically a touch a of reverb and delay.

For channel two, perhaps the DSP is the preamp. I hope someone with far greater knowledge will jump in here.

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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Mon Dec 17, 2012 8:46 pm
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So riddle me this...
Upon opening up my two Super Champs (a head and a combo) I noticed an interesting thing. The tubes in the head, both the pre-amp and power tubes, are marked "premium", not so in the combo (the combo I have is the Tan/Oxblood FSR model). The preamp tube from the combo is a Sovtek, there's no other mark on the preamp tube from the head.

So Alan, or whoever else is in the know, what are GT premiums vs. "regular" GT's? It no matter either way but after poking around on the web, somebody said that some of the GT's are actually JJ's, which is what I just swapped out the GT's for. Did I just swap out one JJ for another? I wish I'd A/B'd them before the swap so I could have compared the new tubes for the old, but I didn't take the time to do so. In any case, the new ECC82S's sound great in both the combo and the head. I think the preamp tube in the head might have been bad anyway as the amp would occasionally make some strange tube related noise intermittently and that has now stopped so it needed to be changed anyhow. Also, I keep referring to the 12AX7's and ECC82S's as "preamp" tubes but I still haven't been able to determine if that's their actual function of them in the SCX2.

AH HA! After poking around some more I found this GT conversion chart... I answered my own question (I think). The new JJ tube I just put in my SCX2 head was indeed already a JJ... the same model I just replaced it with...oops. But it did fix the occasional noise I was getting from the amp so no biggie. The preamp tube from the combo was a Sovtek. So why the different tubes in the head and combo formats of the same amp?
Here's the link to the conversion chart I found... http://www.thetubestore.com/Tubes/Groove-Tubes
I have to wonder how many others who've replaced the GT's in various amps considering them to be sub-par, have simply been replacing one JJ tube for another? Inquiring minds want to know.


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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Tue Dec 18, 2012 8:56 am
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Since we are discussing tubes, the SCX2 and I am a tube noob. Let me ask a question that is sure to embarrass me. If one buys a matched pair of 6v6's from a reputable supplier, can one get away with just dropping them in the SCX2 without biasing? :lol:

This is an honest query.

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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Tue Dec 18, 2012 12:52 pm
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Hmm... not sure. It's my understanding that many newer Fender's are biased on the cool side, meaning that the tone isn't what it might be in terms of richness, but with the benefit being much longer tube life. I asked a guy at a local shop about needing to bias my Twin Reverb before swapping tubes (JJ's) and he said to just install them and look at them to make sure they weren't burning too hot...aka, glowing super red. He also said you can tell because the amp will distort more quickly... now, with a Twin, earlier breakup could still be at a volume way beyond normal house-jamming volume.

I just installed them with no rebiasing, watched to see of they were glowing really hot and they didn't. The amp sounds great and I'm content to leave it be now that it's working. Having said that, 95% of folks will tell you rebiasing is an absolute must... I'm sure there's potential for damage if the amp is biased too hot for the tubes or way too cool but in this case it worked out just fine. There's a lot of chatter to be found in various forums, etc, regarding the need to rebias your amp... to be absolutely safe you probably should but my guess is that you can probably get away with not doing it and be fine.


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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Tue Dec 18, 2012 1:51 pm
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JamGuy wrote:
Hmm... not sure. It's my understanding that many newer Fender's are biased on the cool side, meaning that the tone isn't what it might be in terms of richness, but with the benefit being much longer tube life. I asked a guy at a local shop about needing to bias my Twin Reverb before swapping tubes (JJ's) and he said to just install them and look at them to make sure they weren't burning too hot...aka, glowing super red. He also said you can tell because the amp will distort more quickly... now, with a Twin, earlier breakup could still be at a volume way beyond normal house-jamming volume.

I just installed them with no rebiasing, watched to see of they were glowing really hot and they didn't. The amp sounds great and I'm content to leave it be now that it's working. Having said that, 95% of folks will tell you rebiasing is an absolute must... I'm sure there's potential for damage if the amp is biased too hot for the tubes or way too cool but in this case it worked out just fine. There's a lot of chatter to be found in various forums, etc, regarding the need to rebias your amp... to be absolutely safe you probably should but my guess is that you can probably get away with not doing it and be fine.


Thanks for your answer. I tend to think in a similar vein. I am on my Winter (lean) budget and I could swing a new pair of tubes but not them and a bias kit. I acknowledge the wisdom of rebiasing, but wondered as to the hazard of not doing so. It would seem that with a cooler running amp, I might cheat myself of some sweeter sound, but not create an equipment damaging issue. In the new year I will seek out some fresh work. I do own a deluxe reverb in NY that I would not think of re-tubing without biasing. From what I have read, they really push those 6v6's past what is recommended with the plate voltage. I have a learning curve in front of me, but I have done very well in educational pursuits. What I like, I devour.

Another interesting piece of info I picked up was at Billy Penn's 300guitars site. He was writing about a mod he was doing on the champ 600. SO, we all know draining the caps (capacitors) is crucial to safe working with the actual electronics. He had this little blurb:
Quote:
Start by discharging the filter caps. Turn the unit on, plug in your guitar, turn up the amp, strum the open strings and let them ring while shutting off the amp. When the notes die out, the caps have used their charge.


If correct, I like that method.

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I have some Fender gear.
(Telecaster, Jaguar, Villager 12 String. Mustang l, Bronco, SCX2, Rumble 350, HRDIII, Rumble 2x8 Neo Cab)
http://www.reverbnation.com/modwiz


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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Sat Jun 17, 2017 1:23 am
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I have a background in electronics, and I've downloaded the schematic. The preamp is a mixture of analog solid state and digital. The 12AX7 is the driver and phase splitter for the power tubes. Fender does their good name and their customers a disservice by marketing it as a tube preamp.


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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Mon Jan 08, 2018 1:58 am
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are the GT-6V6-C groove tube in the x2....just rebranded jj's 6V6'S
the tube depot seem to suggest that!, also stating the GT12ax7R = Sovtek 12AX7-WA
Any comments on this

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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Tue Jan 09, 2018 6:10 am
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Virtually all Groove Tubes are rebranded tubes. The GT6L6-GE is the only tube they actually assemble themselves.

Groove Tubes does test and match the tubes. But rebranding has always been their business model. In the '80s and '90s they went around to electronics wholesalers and bought all the NOS tubes they could find. Tested, matched, then relabeled them. So they were rebranded RCA's, Sylvania's, GE's, Raytheon's, real Telefunken's, real Mullards, etc.

Those big stocks of golden age tubes are long gone. So now they buy from current manufacturers like JJ and Reflektor.

BTW, Sovtek is one of New Sensor's brand names. Sovtek rebrands tubes made by Reflektor.

There are only 5 or 6 tube factories remaining in the world. New Mullards, new Telefunkens, TAD's, Ruby's are all rebranded.


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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Fri Feb 16, 2018 8:32 pm
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The single 12AX7 for the phase inverter sound plausable but so does the pre tube.

I always thought the AX7s had 2 sides to the tube they couldnt split the tube could they?

Run one side as a preamp and the other side for the PI ?


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Post subject: Re: What's the actual function of the 12ax7 tube in the SCX2
Posted: Mon Feb 19, 2018 1:02 pm
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The 12AX7 is a phase inverter. The Pre-amp is Digital and solid state as mentioned.


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